High voltage semiconductor devices such as diodes, transistors and insulated gate bipolar transistors usually include heavily doped semiconductor areas so as to define a p-n junction. A p-n junction is one of the basic building blocks of semiconductor technology. For high voltage devices such a junction comprises three regions: a p-doped region which has a high concentration of acceptors, acting as ‘holes’; an n-doped region which has a high concentration of donors, and a drift region separating the n- and p-doped regions. The drift region is also known as a lightly-doped region or voltage-supporting region. The transport of charge carriers through the drift region indicates whether the device is switched on or off. The p-n junction depletes in the off-state (i.e. charge carriers are swept out) and the drift region insulates the p-doped region from the n-doped region, thus supporting the off-state voltage. Many high voltage devices can carry voltages of 60 V and higher.
FIG. 1 shows a high voltage junction 7 represented by a PIN junction. However, other configurations are possible such as RESURF and Super Junctions. The junction 7 shown in FIG. 1 has three regions: the p-doped region 7a, the drift region 7b and the n-doped region 7c. The drift region separates the p- and n-doped portions. FIG. 1 shows a theoretical illustration, since in this example the junction is suspended in air or vacuum 4.
FIG. 2 shows the junction 7 of FIG. 1, wherein a voltage difference V1 is applied across the p-n junction. The electrical potential lines 8 in air or vacuum are homogeneous in the drift region 7b and spread out slowly in vacuum 4. There are no regions in which the density of the electrical field lines increases significantly with respect to the drift region 7b. 
However, in any practical application, in particular in ICs, the junction will not be suspended in air or vacuum, but will be surrounded by other semiconductor devices.
FIG. 3 shows the junction of FIG. 1 embedded in a dielectric region 5 and surrounded by a semiconductor region 2 instead of being suspended in vacuum or air. The semiconductor region 2 is normally held at a low voltage. The p- and n-doped regions are separated from each other by a width 7w. The p-n junction is at a distance 5w from the semiconductor region 2 in the direction indicated in the figure.
In the horizontal plane, the junction is surrounded by the dielectric region 5 as shown in FIG. 3. In vertical direction, the junction is mounted on an insulating layer (not shown). This configuration is called silicon on insulator (SOI).
FIG. 4 shows the junction and surrounding regions of FIG. 3 including the electrical potential lines. The potential lines wrap around the higher potential region 7c causing an electric field peak at the critical n+/drift/dielectric interfaces, indicated by ‘A’ in FIG. 4. The electric field peak causes charge carriers inside the doped portion to accelerate towards the interface. If the acceleration of the carriers is large, they will leave the p-n junction and enter the dielectric material from which they will not be able to return. This effect is called carrier injection into the dielectric material and is undesirable, since it will alter the device's electrical characteristics and parameters such as breakdown voltage and on-resistance. Depending on the electric field and current level this may lead to short-term failure or long-term parameter drift, which degrades reliability.
The distance 5w between the sides of the p-n junction where the potential lines leave the drift region and the surrounding semiconductor material 2 is in one case made to be about the same as, or larger than, the width of the drift portion 7b, in order to reduce the electric field peak at the interface. However, even when the distance 5w is large compared to the dimensions of the p-n junction, there will be some bending in the potential lines and an electric field peak because the surrounding region 2 is held at a low voltage, for example 0V. A large width 5w causes the combined p-n junction and dielectric material to become larger and take up more space.